2006-03-28

Last Sunday, here in the UK, we turned our clocks forward an hour from GMT to BST. This was a bit of a significant event for my weather station setup because, when I first started work on the tools that would pull the data off it and present it on the website, I decided that local time (rather than Universal Time) would make more sense. For reasons I won't bore anyone with it was important that I selected one or the other as the primary method of dealing with time and given that weather data lends itself more to how we portion up our days local time was the choice that won.

There were a handful of minor problems, all of them were to do with presentation and, in each case, they were to do with the fact that I'd written some code that assumed UT rather than GMT/BST. As far as I can tell each of these issues is now dealt with and things are displaying correctly.

Of course, the real test will be the switch from BST to GMT. Skipping forward an hour is one thing, skipping backward an hour is quite different.

The last 24 hours or so has been very windy over the UK but you wouldn't really know it by looking at the data from my weather station. The maximum speed I've recorded in that time is 19.5mph — I'm pretty certain that we've had sustained speeds greater than that and we've easily had gusts over that.

I've read in a few places (including a comment someone placed here) that the vane/anemometer that comes with the WS3600 heavily under-reports wind speeds and this would appear to be the case.

Watching the vane yesterday I'm wondering of the problem all comes down to it using an impeller that has to be turned into the wind by the vane. Consider this graph of wind direction from the second half of yesterday:

I would have said that yesterday afternoon and late into the night had far stronger wind speeds than during the first half of the day. Notice how there is no clear direction in the image above? I noticed this while watching the vane, the stronger the wind the more it gets blown around, the less it faces into the wind. If it isn't facing into the wind the impeller isn't going to be getting the full force of the wind and will generally under-report the speed.

Without actually looking into this (something I've not done but will at some point) I'm guessing that a separate cup anemometer and vane setup is a better solution than an impeller attached to the vane.

To be fair, I've not got the vane placed in the most ideal location and this might also have something to do with what I'm seeing. While the vane is above all the nearest obstacles there's no useful way I can have it high enough that it's above the height of my house and the house next to me. It's not close to them, but I'm sure there's still plenty of turbulence off them. Again, in this sort of setup I'm guessing a separate cup anemometer would be a better choice.

2 comments:

You are definitely getting turbulence from the nearby houses. Don't you know that the Met.Office recommend the Wind thingy to be in the middle of a field or something like that to get accurate readings? :-) If you can, I would ay to put it on your chimney though watch for temp. varitations due to fire going or boiler etc. The temp. sensor needs permanent shade but I have found that it all works a treat. I occasionally oil the wind speed sensor and can read low speeds no prob.

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By night (and weekends and bank holidays) I'm a software developer, photographer and general geek who, when he can be bothered to get out of his office chair, enjoys a bit of observational astronomy too (sadly mostly on hold for now).

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